Sun injury on apple fruit: Physiological, biochemical and molecularadvances, and future challenges⋆
Luis Morales-Quintanaa, Jessica M. Waited, Lee Kalcsitsd, Carolina A. Torresd, Patricio Ramosb,c,⁎
aMultidisciplinary Agroindustry Research Laboratory, Instituto de Ciencias Biomédicas, Universidad Autónoma de Chile, Talca, Chileb Instituto de Ciencias Biológicas, Universidad de Talca, Talca, ChilecNúcleo Científico Multidiciplinario-DI, Universidad de Talca, Talca, Chiled Tree Fruit Research and Extension Center, Department of Horticulture, Washington State University, Wenatchee, WA, United States
Climate change negatively influences many human activities and one of the most affected is agriculture. In theapple industry, water availability, elevated temperatures and altered phenology will transform fruit productionin traditional growing regions. Extended periods of intense solar radiation and high temperatures during thegrowing season cause problems in fruit quality increasing losses and reducing sustainability and profitability.Photooxidative and heat stress stimulate sunburn development on apple fruit in the field growing under in-creasingly stressful conditions. In particular, apples growing in semi-arid conditions are frequently exposed tohigh solar irradiance and elevated temperature during the growing season that promote the development of sun-related skin disorders. Furthermore, regions that have traditionally not faced sunburn pressure may begin toexperience losses in this area. Apple cultivars differ in their susceptibility to sun damage, which is evidenced, inpart, by the timing of symptom development and severity. Some studies attribute genotypic variation to phy-siological and morphological differences while others do to antioxidant-related metabolic differences betweenthem. Here, we discuss the physiological and molecular progress and gaps in knowledge of sunburn damage andthe development of sunburn resistance in apple fruit. This information will help develop stronger sunburn mi-tigation strategies and enhance breeding efforts to address challenges associated with sunburn in apple pro-duction.
1. Introduction
In apples (Malus domestica Borkh), stressful conditions such as highirradiance, elevated temperatures, and low relative humidity stimulatethe development of physiological disorders including sunburn(Schrader et al., 2001, 2003) and watercore (Ferguson et al., 1999).Climate change will affect global agricultural production and specifi-cally for apple, longer periods of extreme solar radiation and hightemperatures during the growing season can reduce fruit quality andincrease losses. Sun injury or sunburn symptoms range from whitepatches to dark brown regions developing on the fruit, depending oncultivar and environmental conditions. Affected fruits can show otheranomalies in their skin as lenticel marking and postharvest sunscald(Hernandez et al., 2014). Also, fruit can be more prone to pathogenattack in the affected area (Racskó et al., 2005). These sun-relateddisorders can strongly affect fruit quality and, as a consequence,
decrease market value (Brown, 2009).Natural defense mechanisms provide some degree of fruit protec-
tion, and cultural practices have also been used to reduce sun injury.Physiochemical properties such as the thickness of the epicuticularlayer, the composition of waxes and pubescence (Wünsche et al.,2004a), accumulation of antioxidant compounds and photoprotectivepigments (Felicetti and Schrader, 2009) can all affect the susceptibilityto sunburn. Horticultural strategies such as the application of Kaolin-based film to reflect the solar radiation on fruit surface (Gindaba andWand, 2005; Glenn et al., 2002; Wünsche et al., 2004b) and shade netson top of the trees in orchards to prevent the excessive radiation(Gindaba and Wand, 2005; Iglesias and Alegre, 2006) can be used toprevent sunburn. Nevertheless, the generation of sun-related physiolo-gical disorders in fruit is a complex and understudied process. Thisproblem requires further research into the environmental and physio-logical processes that occur prior to and during sun injury development
https://doi.org/10.1016/j.scienta.2019.108866Received 19 May 2019; Received in revised form 16 September 2019; Accepted 17 September 2019
⋆Main message: The development of resilient apple cultivars resistant to sunburn requires a better understanding of the physiological, biochemical and molecularcontrols regulating sunburn development in apple.
and more importantly, biochemical changes that may contribute toresistance to environmental conditions that cause sun-related disordersin fruit.
2. Characterization of sunburn development in fruit on the tree
Sun injury in fruit is caused by photo-oxidative stress in chlorophyll-containing tissue when exposed to intense solar irradiation and ele-vated temperatures during the growing season (Naschitz et al., 2015;Rabinowitch et al., 1974, 1983; Torres et al., 2006). Excess light energycan overwhelm the light absorption capacity of fruit. When light-energyabsorbed by the tissue exceeds its photosynthetic capacity, reactive-oxygen-species (ROS) can form causing photo-oxidation that can resultin the manifestation of sunburn symptoms (Ma and Cheng, 2003, 2004;Torres et al., 2006) (Fig. 1).
Sun injury symptoms vary upon cultivar and environmental condi-tions from light to dark brown areas and necrosis with or without tissuebleaching (Felicetti and Schrader, 2008). Schrader et al. (2001) foundthat sunburn symptoms on apples developed when, in addition to highlight, fruit reached a threshold temperature that they called minimalFruit Surface Temperature (FST). Higher the temperature, the less timerequired to induce damage of different degrees. Fruit Surface Tem-perature threshold varied with cultivar, where ‘Cameo’ and ‘Honey-crisp’ showed the lowest threshold (46 °C) and ‘Cripps Pink’ the highest(49 °C) (Schrader et al., 2008). Tissue bleaching caused by chlorophyllphotooxidation can be more important in de-acclimated fruit. Schrader
et al. (2008) found that this type of damage, which they called pho-tooxidative sunburn (PS), can appear at FST below 31 °C under one dayof high irradiance. While PS is known to develop from sudden exposureto high light conditions, the threshold for shading and sun exposure isstill relatively unknown. The adoption of netting systems (Mupambiet al., 2019) with varying shade factors (Mupambi et al., 2018a,b) andwith the capacity to be rapidly deployed and retracted may increase therisk of photooxidative sunburn events. To prevent these issues, a betterunderstanding of the impacts of adaptation of fruit to light is requiredincluding thresholds limits for where reductions in light reduce inducedacclimation responses.
3. Additional factors affecting sunburn development
Several orchard-related factors also contribute to sunburn devel-opment, such as cultivar and rootstock, row orientation, tree structure,and horticultural practices that affect fruit exposure to direct sunlight(Parchomchuk and Meheriuk, 1996). High-density plantings withsmaller trees are more susceptible to sunburn injury because of greaterfruit exposure than in traditional lower density plantings with largertrees (Parchomchuk and Meheriuk, 1996). Dwarfing rootstocks limitvegetative vigor and increase light penetration into the canopy in-creasing susceptibility to sun-related damage (Middleton et al., 2002;Racskó et al., 2005; Gonda et al., 2006; Racskó et al., 2009). Row or-ientation is also an important factor affecting sunburn incidence inorchards where north-south orientations have higher sunburn incidence
Fig. 1. Environmental, physiological and biochemical factors involved in sun injury development on apple and other fleshy fruits.As sun injury develops on sun-exposed fruit, photooxidative, thermal and osmotic stress drive phenotypic changes not only on fruit surface but also underneath deepinto the cortex tissue. Visually, fruit peels start discoloring due to chlorophyll degradation and later different levels of bronzing (enzymatic browning) occurssometimes followed by thermal damage and cell rupture. During this process cells engage dynamic defense mechanisms signaled by ROS and stress-related phy-tohormones leading to enhanced antioxidant metabolism and thermal dissipation mechanisms, as well as osmoprotectants synthesis and accumulation. Some species,such as apples, within minutes upregulate the phenylpropanoid pathway leading to flavonoids accumulation, as well as anthocyanins and monolignols in a lesserextent, providing static protection acting as solar screen.
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on the west side while east-west orientations have higher incidence onthe south side of the trees (Barber and Sharpe, 1971). Orientation isparticularly important during the critical afternoon period when sun-burn pressure is greater (Barber and Sharpe, 1971). Mineral nutritioncan also have direct or indirect effects on sunburn development (Racskóand Schrader, 2012). Racskó et al. (2006) reported a negative corre-lation between the amounts of nitrogen applied and sunburn suscept-ibility of apples where enhanced vigor can provide protection for fruitfrom sunburn. However, over application of nitrogen is not a reliabletool for sunburn control because of negative effects on fruit quality andstorability. Cultural practices that can be used to limit fruit sunburndevelopment include evaporative cooling (Gindaba and Wand, 2005),protective sprays (Racskó and Schrader, 2012) and netting (Kalcsitset al., 2017; Mupambi et al., 2018a,b; Mupambi et al., 2019). However,these strategies are not completely effective in controlling sunburn andcan have negative consequences or significant costs associated withthese practices that limit adoption or effectiveness. It is critical that,moving forward, new cultivars that are being developed need to bemore tolerant to high temperatures during fruit development.
Some cultivars are more susceptible to sunburn than others in thesame edaphoclimatic conditions (Meyer, 1932; Moore and Rogers,1943). Studies conducted in several world regions have shown that‘Granny Smith’ and ‘Jonagold’ are highly susceptible to sunburn(Racskó et al., 2005; 2011; Sibbett et al., 1991; Van den Ende, 1999);‘Fuji’, ‘Golden Delicious’, ‘Braeburn’, ‘Boskoop’, and ‘Delicious’ aremoderately sensitive (Carbó et al., 2005; Racskó et al., 2005), whereas‘Cripps Pink’, ‘Idared’, and ‘Topaz’ as the less susceptible (Van denEnde, 1999; Gindaba and Wand, 2005; Racskó et al., 2005, 2011; Lebeand Schulte, 2008). As indicated by Racskó and Schrader (2012), early-ripen cultivars (e.g., ‘Gala’) display strong sunburn damage some years,but not other years. In addition, a study conducted in several applecultivars showed no differences in fruit peel photosystem efficiencyunder heat stress at 50, 95 and 150 days after full bloom (Hengari et al.,2014a, b). Furthermore, in fruit exposed to UV-B light, PSII photo-chemical efficiency ratio decreased in green varieties but remainedstable in red varieties, supporting the protective role of anthocyaninsagainst high light conditions (Hengari et al., 2014a, b). In the last twodecades, the transition from three-dimensional trees to simplified two-dimensional structures with dwarfing rootstocks has increased sunburnrisk (Hampson et al., 2002). These changes coupled with a recent his-tory of elevated summer temperatures has led to the presence of sun-burn in regions that traditionally have not had issues with this disorder(Reig et al., 2019). It will be important that new cultivars developed areless susceptible to sunburn. However, to achieve these goals, a betterunderstanding of the underlying physiology conferring resistance tosunburn in apple fruit needs to be developed.
4. Biochemical response mechanisms of apple fruit to sunlight andradiation
Apple fruit exposed to excessive solar radiation switch on a complexdefense program that prevents molecular damage and minimizes tissueinjury. The stimulation of this defense system can induce morphologicalchanges, pigmentation, and finally, the development of sunburn
symptoms and alteration of fruit quality. When the excitation energyfrom sunlight exceeds the fruit’s capacity to absorb light, thermal dis-sipation, or scavenging of free radicals by antioxidants, photooxidativedamage can develop (Bertamini et al., 2004; Torres et al., 2006). Da-mage to the photosynthetic apparatus occurs by the denaturation ofphotosystem II proteins, the photosynthetic electron transfer system,and Calvin cycle enzymes (Cheng et al., 2008; Smillie, 1992). Bio-chemical defense mechanisms to combat this damage can include an-tioxidant metabolites, such as ascorbic acid (AsA) and glutathione(GSH), and antioxidant enzymes such as ascorbate peroxidase (APX),dehydroascorbate reductase (DHAR), monodehydroascorbate reductase(MDHAR), glutathione reductase (GR), catalase (CAT) and superoxidedismutase (SOD) (Cheng et al., 2008; Torres et al., 2006). The con-centrations of phenolic compounds important for defense are also in-duced by stress (Felicetti and Schrader, 2008). During the developmentof sunburn, chlorophyll and carotenoid concentrations decrease, andxanthophyll concentrations increase (Cheng et al., 2008; Torres et al.,2006). Recently, a comprehensive analysis of several antioxidant me-tabolites, enzyme activity, and their respectively transcript abundancewas assessed in apple fruit showing severity of sun-injury symptoms inthe tree and after several days of cold storage (Hernandez et al., 2014).In this study, it was reported that ‘Granny Smith’ apple fruit tissuepresenting symptoms of sunburn had minimal concentrations of anti-oxidant metabolites (AsA or GSH) or antioxidant enzymes (APX, DHAR,MDHAR, GR, CAT) to cope with oxidative postharvest stress. Zupanet al. (2014) assessed phenolic composition and peroxidase activity inthe peel of sun-exposed fruit for ‘Braeburn’ and ‘Golden Delicious’apple. Here, they reported strong phenolic compounds and peroxidaseactivity in response to photooxidative stress, which could be relatedwith the peel color of apple (Table 1). Interestingly, genotypic differ-ences among apple cultivars in the accumulation of heat shock proteins(HSP) in the peel of apple under high temperature in the field has beenreported (Fig. 2) (Ritenour et al., 2001). HSPs are molecular chaperonesthat provide tolerance to high temperatures, avoiding denaturation andaggregation of target proteins and helping protein refolding. Thesedifferences among cultivars may correspond to some degree of sunburnresistance.
Table 1Apple peel relative lightness of colors (L*) (mean ± SE) and hue angel (h◦) (mean ± SE).
Sunburn Shaded side of sunburned Healthy sun exposed side Shaded side of healthy
L‘Golden Delicious’ 53.0 ± 1.2 a 61.5 ± 0.4 b 61.9 ± 0.8 b 61.6 ± 0.3 b‘Braeburn’ 57.4 ± 1.0 a 59.9 ± 0.4 b 59.6 ± 0.6 b 58.7 ± 0.5 abhº‘Golden Delicious’ 73.5 ± 1.4 a 106.2 ± 0.2 c 100.4 ± 1.1 b 106.0 ± 0.5 c‘Braeburn’ 75.5 ± 2.1 a 105.6 ± 0.2 c 101.4 ± 0.9 b 105.7 ± 0.2 c
Table adapted from Zupan et al., 2014.
Fig. 2. Immunoblot of protein from apple peel of different cultivars exposedand non-exposed to direct sun light in field. Blot was tested with antibodiesagainst human HSP70 and HSP 18.1. Image adapted from Ritenour et al.(2001).
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5. Molecular mechanisms involved in sunburn developmentresponses
The activity of enzymes involved in sorbitol and glucose metabolismwere reported to be significantly different between sun-exposed andshaded apple tissue (Li et al., 2013). This study also reported changes inflavonoid and anthocyanin biosynthesis-related enzymes in addition tointermediary metabolites during sunburn development in apple fruit (Liet al., 2013). Identifying the main components of the transcriptionalnetwork will be critical for identifying the genetic differences under-lying the physiological differences in sunburn susceptibility amongapple cultivars, and would inform the development of new cultivarswith enhanced resistance to sunburn.
It is well known that members of the MYB transcription factor (TF)family are involved in transcriptional regulation of the phenylpropa-noid pathway in plants, including flavonoid and lignin biosynthesis(Dubos et al., 2010). Knowledge about the regulatory specificity of MYBTFs and their interactions with other factors which control flavonoidbiosynthesis is important for understanding the genetic pathways ac-tivated by each TF. In Arabidopsis thaliana, accumulation of anthocya-nins and their precursors have been suggested to be stress-dependent,and probably mediated by stress-specific TFs (Kovinich et al., 2014). Inapple, the TF MdMYB10, and its allelic genes, MdMYB1 and MdMYBAhave been reported to control apple red color development through thetranscriptional regulation of structural genes in the anthocyanin bio-synthetic pathway (Ban et al., 2007; Espley et al., 2007; Espley et al.,2009; Wang et al., 2011). Apple skin patterning has also been asso-ciated with differential expression of MYB10. Both ‘Honeycrisp’ and‘Royal Gala’ had higher mRNA levels of MdMYB10 and biosyntheticgenes MdCHS, MdCHI, MdF3H, MdDFR1, MdLDOX, and MdUFGT in redstripes compared to green stripes (Telias et al., 2011).
Despite the economic importance and extensive understanding ofthe environmental conditions leading to sunburn development, thegenes and their associated molecular pathways associated with sunburndevelopment in apple are still poorly described. However, recentlystudies have started to describe the molecular changes associated withsunburn development in fruit. For example, Feng et al. (2013) exposedfruit to full sun by turning fruit 180° about one week before harvest todetermine the expression of key genes involved in anthocyanin synth-esis in response to sunlight exposure, and their relationships with thelevels of anthocyanins and other phenolic compounds. The sun-exposedpeel had higher expression levels of MdMYB10, and seven structuralgenes in anthocyanin synthesis (MdPAL, MdCHS, MdCHI, MdF3H,MdDFR1, MdLDOX, and MdUFGT), and higher levels of anthocyaninsand flavonols compounds compared to the shaded peel for two differentcultivars: ‘Fortune’ (red peel) and ‘Mutsu’ (yellow/green peel) (Fenget al., 2013). Interestingly, when the shaded peel for the fruit was ex-posed to full sun for both cultivars, significant up-regulation of theexpression of MdMYB10 and all seven structural genes was reported.Consequently, this led to higher levels of anthocyanins, flavonols, andtotal phenolics than in either the shaded peel or sun-exposed peel ofcontrol fruit (Feng et al., 2013).
Lignin accumulation occurs not only in trees and model plants, butalso in fleshy fruit as loquat (Eriobotrya japonica Lindl.) (Cai et al.,2006) and mangosteen (Garcinia mangostana L.) (Kamdee et al., 2014).For fruit, accumulation is induced by elevated temperatures and me-chanical stress (Kamdee et al., 2014; Xu et al., 2014). In strawberry,cultivar-specific differences in lignin accumulation were reported (Ringet al., 2013). MYB TFs have been linked to flesh lignification. In loquatfruit, two MYB transcription factors were reported to regulate loquatflesh lignification, with activation by EjMYB1 and repression byEjMYB2 (Xu et al., 2014). Recently, Wang et al. (2016) demonstratedthat EjMYB8 is also able to transcriptionally regulate the lignification ofloquat fruit after temperature-induced stress conditions.
6. Hormones and damage in apples exposed to solar radiation
Plants are phenotypically plastic in response to environmental sti-muli. Most commonly described responses include growth and devel-opment through the action of phytohormones (Wolters and Jürgens,2009). Based on these generalized descriptions, Torres et al. (2013)suggested that photooxidative stress responses in apple fruit could bemodulated by ethylene, which has recently been in apple (Torres et al.,2017). Additionally, auxin (indole-3-acetic acid, IAA), abscisic acid(ABA), jasmonic acid (JA), salicylic acid (SA) profiles were assessedwhen apples were exposed to either photooxidative or heat stress(Torres et al., 2017). For this study, IAA was not directly related to thedevelopment of sun injury symptoms. However, IAA strongly decreasedin sun-exposed tissue in addition to a general decrease during fruitdevelopment. ABA, JA, SA and ethylene concentrations have been re-ported to increase significantly in fruit with moderate damage (Torreset al., 2017). With the exception of ethylene, concentrations of theseimportant phytohormones were unchanged for fruit exposed to solarirradiation (Torres et al., 2017). In Japan, the foliar application of S-ABA, the biologically active form of ABA ([5-(1-hydroxy-2,6,6-tri-methyl-4-oxo-2-cyclohexen-1-yl)-3-methyl-2,4-pentadienoic acid]) re-sulted in a reduction of sunburn incidence in ‘Tsugaru’, ‘Sensyu’, ‘Ya-taka’ and ‘Fuji’ apples cultivars by up to 30% (Iamsub et al., 2008;Iamsub et al., 2009). In contrast, Mupambi et al. (2018a,b) reportedthat foliar applications of S-ABA were ineffective for reducing sunburnin apple in South Africa. Overall, the inconsistent association of specificphytohormones during the development of sunburn symptoms suggeststhat fruit peel responses to high light and temperature involve complexpathways and interactions that need to be described in greater detail.
7. Fruit acclimation to heat and light stress: physiological andmolecular changes
As previously discussed, tolerance to the harmful effects of excesslight and heat varies among different apple cultivars (Gindaba andWand, 2005; Racskó et al., 2005, 2011). This tolerance may, in part, bedue to an enhanced ability to acclimate to environmental stimuli. Manydifferent plant species have been reported to develop memory to stress.This memory provides the platform for acclimation to stress and thusprotection against future potentially damage-inducing environmentalconditions. These types of responses have not been described in apple.For other species, such as alpine blueberry (Vaccinium gaultherioides),exposure to controlled heat events simulating long-term moderate heatwaves (30 °C for 7 days) led to significant reductions in heat-inducedlethality in leaves that were re-heated to determine tolerance (Karadaret al., 2018). The authors additionally reported that growth under lowlight improved heat-hardening in these plants. Studies in both modelsystems and crop plants have uncovered molecular and physiologicalevents involved in temperature memory and acquired thermotolerance(Friedrich et al., 2019; Katano et al., 2018; Yeh et al., 2012). Similar tothe study in alpine blueberry, many of these experiments involve a“priming” stimulus with a sub-lethal temperature, followed by a restand a retreatment with higher temperatures. Upon retreatment, avariety of traits have been measured to assess the acquisition of heattolerance, including seedling viability, membrane integrity and elec-trolyte leakage, chlorophyll accumulation in leaves, organ growth,photosynthetic efficiency, and fruit ripening and grain filling (Yehet al., 2012). Despite the variety of traits and developmental stagesstudied, these investigations have revealed some common responsesand genetic pathways.
Proteins in the Heat Shock Factor (HSF) family, as well as severalsmall Heat Shock Proteins (HSPs), have been implicated in acquiredthermotolerance. Experiments in Arabidopsis were able to show me-chanistic insight into the requirement of two HSPs, HSFA2 and HSA32,for long term acquired tolerance (Charng et al., 2006; Lämke et al.,2016). Additionally, a feedback loop between HSA32 and HSP101 was
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found to improve heat acclimation in rice (Lin et al., 2014). HSFA1 wasshown to be important for acquired thermotolerance, both in plantviability and in fruit ripening in tomato (Mishra et al., 2002), as well asseedling viability in Arabidopsis (Yoshida et al., 2011). Many of theseHSF and HSP members have additionally been described as key playersin ROS signaling and regulation (Katano et al., 2018). Chromatin-basedand epigenetic responses have also been identified as central to stressmemory and adaptation (Friedrich et al., 2019; Lämke and Bäurle,2017). A recent study in potato demonstrated a light-dependent ac-quired thermotolerance response, highlighting expression differences inchromatin-remodeling genes and HSPs, as well as genes involved inhormone pathways, cell-wall modification, protein turnover, and pho-tosynthesis between acclimated and non-acclimated plants (Trapero-Mozos et al., 2018).
Also of potential interest is the phenomenon of cross-priming, wherea sub-lethal stimulus with one type of stress (i.e. cold, drought, salt, orheat) leads to improved tolerance to another stress. For example, re-ports of cold treatment leading to increased heat tolerance have beendescribed in grape, and heat treatments have been used to decreasechilling injury in tomato (Wan et al., 2009; Zhang et al., 2013). Simi-larly, priming plants with chemicals has been used as a way to hardenplants to future abiotic stresses (reviewed in (Savvides et al., 2016)). Aproposed hypothesis to explain that abiotic stresses, as well as thesechemical priming agents, all induce similar physiological and molecularchanges, such as induction of stress-related genes and antioxidants, andprotein modifications. To develop the potential of chemical primingagents or the use of cross priming mechanisms in sunburn risk
assessment in apple will require substantial research into understandingthe conditions that induce stress protection mechanisms (Fig. 3).
In addition to the molecular and physiological components dis-cussed here, many more genes identified in Arabidopsis appear to beinvolved with temperature memory and acquired heat tolerance. Thesegenes may help guide the study of acclimation in crop systems throughthe identification of gene orthologs in these crop species. Moving for-ward, research into the molecular mechanisms underlying acclimationto sunburn-inducing stresses in apple, as well as sunburn development,will help elucidate how fruit acclimate to high heat and lightthroughout the growing season and inform management strategies toavoid solar injury.
8. New strategies for early detection of sun damages on apples
Lack of genetic information regarding the molecular mechanismcommanding photooxidative and heat stress during sun damage onapple fruit highlights the importance to develop investigative frame-works to better understand their regulation. Progress in this arena canbe in breeding programs that are currently in regions that experiencesunburn pressure and areas that are forecast to be impacted by futureclimate change. To support this, several studies have been conducted toidentify and characterize TFs involved in resistance or tolerance todifferent abiotic stresses, with successful results in several crop systems(Gürel et al., 2016). Molecular manipulation of TFs has the potential tobe more effective than modification of an individual gene involved in aspecific stress response, due to the ability to modulate expression of
Fig. 3. Variable temperature regimes are used to investigate the molecular players and mechanisms underlying acclimation and thermotolerance. A) Acclimation andacquired thermotolerance studies often involve raising the temperature to an elevated, but sub-lethal temperature to induce plant defenses and stress memory. Moststudies to date have used a 30–60minute stimulus, however some studied have elevated temperatures up to 7 days. This will be followed by a rest at the controltemperature. Studies investigating short-term acquired thermotolerance will have a short rest of a couple hours. Long-term thermotolerance studies will allow a 2–3day rest. Following the rest, plants are then challenged with an otherwise lethal heat shock stimulus, usually 30–90minutes, but some studies have lengthened thistime to 48 h. Variable rest times are then followed by measurement of the output trait. B) Several examples of characterization of genes involved in acquiredthermotolerance, as well as studies into cross-priming effects. Both HSA32 and HSFA2 are involved in long-term acquired thermotolerance and plant mutants lackingtheir expression are unable to acclimate to heat stress. In tomato, a hot-air stimulus was able to improve ripening and reduce chilling injury in cold-stressed tomatofruit.
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several downstream-genes involved in metabolic pathways associatedwith stress tolerance in plants (Khong et al., 2008; Nakashima et al.,2012).
Torres et al. (2013) performed a study analyzing sun-exposed versusnon-exposed tissues of apples and reported that osmotic stress is alsogenerated under photooxidative and heat stress in the field, evidencedby lower water content and increase sugars as sorbitol and glucose insun-exposed tissue. Recently, a crop water stress index (CWSI) was alsoproposed as an early indicator of sun-damage in fruit without visiblesymptoms (Fig. 4) (Torres et al., 2016b). Through thermal images, theoccurrence of sunburn symptoms in Fuji was predicted to be earlierthan in Royal Gala apples. These responses highlight cultivar-specificresponses to sun-related stress. In addition to CWSI analysis, Vis/NIRfingerprint analysis of fruit surfaces with different sun-exposure wasalso able to predict sun damage in Granny Smith apples (Fig. 3) (Torreset al., 2016a). Both technologies could be useful for early identificationof molecular events commanding physiological responses involved insunburn, prior to the development of visual symptoms.
9. Genomic information and new transcriptomic challenges tosupport future breeding strategies
Understanding the molecular basis that control the responses ofapple fruit exposed to excessive sunlight or temperature, which causesunburn damage, is imperative for the development of new cultivars
that are better suited to hot and dry environments around the world.Identifying the underlying physiology imparting sunburn resistance inthe orchard wll also help in the creation of new protective products ordecision aid systems that can help reduce losses for susceptible culti-vars. The full apple genome that is available today (Velasco et al., 2010)provides opportunities to identify key genetic and molecular mechan-isms contributing to sunburn in apple. Specifically, the analyses ofpromoter regions of differentially expressed genes between susceptibleand tolerant cultivars in order to identify common Cis-regulatory ele-ments will be helpful to distinguish TFs from RNAseq data, which couldbe critical for regulating the molecular response that occurs prior tophysiological disorder development. With this information, molecularmarkers could help design new cultivars in breeding programs targetedto apple production in semi-arid growing conditions such as thosefound in Chile, WA state-USA, South Africa, Argentina, Australia andIsrael, among others.
Declaration of Competing Interest
The authors declare no competing financial interests.
Acknowledgments
PR acknowledges ‘Núcleo Científico Multidisciplinario’ fromUniversidad de Talca. LK work was also supported by the USDANational Institute of Food and Agriculture, Hatch/Multi-State project1014919. LK and JW were funded through funding from theWashington Tree Fruit Research Commission (AP-18-100). The fundershad no role in the study design, data collection and analysis, decision topublish, or preparation of the manuscript.
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Fig. 4. A) Thermal of infrared image used to calculate the crop water stressindex (CWSI), an early predictor of sun damage on apple surface. B)Classification of sun-injured fruits to calculate damage in peel by Vis/NIRspectra. C) Vis/NIR spectra of fruit from different sun exposure categories(classes 0–4). Images adapted from Torres et al. (2016a, 2016b).
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